High reliability solutions for center bond lead μBGA® packages

As the compliant layer construction of μBGA packages improves second level reliability by dramatically reducing stresses to the package/printed circuit board solder joints. However, these stresses caused by thermal mismatch are shifted to the bonded leads. The majority of package or system failures from thermal cycling are due to lead fatigue fractures. It has been also recognized that peripheral and center bond μBGA leads fatigue differently during thermal cycling. A peripheral lead experiences movement from thermal expansion or contraction in all three axes, x, y and z. Sufficient lead slack is the dominant parameter that controls package reliability. The center bond lead differs from its peripheral counterpart in that z-axis motion is dominant during thermal cycling because center bonds are very close to the package neutral point and therefore the x and y displacements are essentially negligible. Fatiguing is mainly caused by z-axis displacement that compresses and raises the lead even if positive slack is maintained during temperature cycling. Because displacement in package height is the dominant thermally induced lead stress in CBL packages, lower spacers and lower CTE encapsulants were introduced and investigated for package reliability improvement. Significant reliability improvements were observed as spacer height decreased, especially in the range off 100-200 μm. A new lead bonding technique employing wirebond software is developed to break the lead notch and bond at spacer heights lower than 125μm, which couldn´t be done with standard μBGA bond motion. Low CTE silicone encapsulants are then introduced and evaluated for CBL applications. Obvious reliability improvements were achieved for parts encapsulated with lower CTE materials.